In the recent twenty years or so, the power electronic technology has obtained a rapid development, and has been widely applied to the fields of electrical power, chemical engineering and communications. The electrical power apparatus generally receives the electrical power from the grid through the rectifiers. A typical rectifier is a nonlinear circuit including diodes or thyristors. Due to the rectifier, lots of current harmonics and reactive power are generated in the grid, which pollute the grid and become a public nuisance. The electrical power apparatus has become the main harmonic sources of the grid. The active approach is generally used to curb the generation of the harmonics, wherein a new generation of high-performance rectifiers are designed which have the features of sinusoidal input current, low amount of harmonics and high power factor. Recently, the PFC circuits have attained a great development, and become an important research direction of the power electronics.
The boost circuit is one of the most frequently used PFC circuits, which possesses the advantages of simple configuration and small input EMI filter etc., but is only suitable for the occasions where the output voltage is larger than the input voltage. For those application occasions that the input voltage is fluctuating in a wide range, e.g. the input voltage is larger than the output voltage in some occasions; a single stage boost circuit is not suitable. Thus, the PFC circuits having the buck-boost configurations are widely used in the aforementioned occasions, and the input current could quite nicely tracking the input voltage and has a relatively low THD.
FIG. 1 is a circuit diagram of a conventional single-phase three-level buck-boost PFC circuit, which has diodes D1-D2 and D11-D14, switches S11-S14, inductors L11-L12, input power source Vin and output capacitors C1-C2, and outputs a positive voltage +Vo between a first terminal and a neutral point and a negative voltage −Vo between a second terminal and the neutral point, wherein the first terminal is a terminal of C1; the second terminal is a terminal of C2 and the neutral point is the connected terminal of C1 and C2. The output voltages +Vo and −Vo could be any values theoretically.
The upper and lower portions of the circuit as shown in FIG. 1 are fully symmetrical. The diode D1 conducts, a current flows through S11 and the upper half of the circuit operates during the positive half cycle of the input voltage Vin, that is to say, Vin (an instantaneous value) is larger than 0. When the input voltage Vin (the instantaneous value) is smaller than 0 (the negative half cycle of Vin), the diode D2 conducts, the lower half of the circuit operates and a current flows through S12 back to the electrical power network. Thus, the controls of the whole circuit in the positive half-cycle and in the negative half-cycle of the input voltage are respectively independent. For simplicity, the circuit operating in the positive half-cycle is analyzed as an example, and those operating in the negative half-cycle can be analyzed by the same token.
When the input voltage Vin (the instantaneous value) is larger than 0, the circuit of FIG. 1 is equivalent to that shown in FIG. 2, and the operating modes of FIG. 2 are analyzed as follows.Vo>√{square root over (2)}Vin  a.
When the output voltage is larger than the peak value of input voltage (√{square root over (2)}Vin is the voltage peak value of Vin), the output voltage is always higher than the input voltage which is shown in FIG. 3(a). Under the condition shown in FIG. 3(a), the circuit operates under a boost mode, that is—S11 is on constantly, and D11 is not conducted.Vo≦√{square root over (2)}Vin  b.
When the output voltage is smaller than the peak value of input voltage, the converter will switch between the buck mode and the boost mode as shown in FIG. 3(b). In FIG. 3(b), the timings when the input voltage intersects the output voltage are α and π−α(0<α<π/2). During the intervals (0,α) and (π−α,π), the output voltage is large than the input voltage; S11 is on constantly; D11 is off constantly; and the circuit operates under a boost mode. During the interval (α,π−α), the output voltage is smaller than the input voltage, S13 is constantly off; D13 is constantly conducted; and the circuit operates under a buck mode.
According to the above-mentioned analysis, D13 is constantly conducted when the circuit operates in the buck mode. Since the forward voltage of D13 is around 1.2 V under the full-loaded condition, the conduction loss consumes of D13 is quite large. When the output voltage is kept constant, the higher the input voltage, the longer the circuit operates in the buck mode, and the larger the conduction loss of D13. Thus the efficiency of the whole system is influenced greatly.
When the input voltage is during its negative half cycle, that is the input voltage is less than 0, the input voltage Vin is boosted when it is larger than the output voltage Vo; and the input voltage is bucked when it is smaller than Vo.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived single-phase and three-phase dual buck-boost/buck power factor correction circuits and a controlling method thereof.